This invention relates to inorganic mesoporous molecular sieves of MCM-41 type. This type of material has high thermal stability, well defined uniform pore openings in the mesopore range, 15 to 100 xc3x85and possesses very high surface area ( greater than 1000 m2/g). The pore openings also can be tailored during the synthesis process by using surfactant groups of suitable chain length.
These materials attracted considerable attention in recent years for their potential use in catalysis. Due to the large pore openings catalytic conversion of bulkier molecules as encountered in the cracking of heavy crude oils or in the manufacture of fine chemicals and pharmaceuticals can be performed easily on these mesoporous molecular sieves.
Incorporation of metal atoms to the framework of mesoporous molecular sieve can generate active sites for catalysis. Incorporation of aluminum generates acidity in the framework. Other elements like vanadium, titanium can also be incorporated in the framework.
The main drawback of the application of MCM-41 molecular sieve in catalysis is its low stability in presence of water vapor and in many reactions water is formed as by-product during the reaction. Also many catalytic processes have to go through very harsh conditions. The practical application of these mesoporous molecular sieve will be slow unless their poor hydrothermal stability is improved. Improvement of the hydrothermal stability of mesoporous molecular sieve has been reported to be achieved by controlling the pH of the gel during the synthesis conditions. However, the described method is exhausting and requires multiple adjustment of pH and hydrothermal treatment of the gel.
Accordingly, it is an object of the present investigation to provide MCM-41 mesoporous molecular sieve which possesses high hydrothermal stability.
Another object of -the invention is to provide a simple and economical method by which hydrothermally stable MCM-41 molecular sieve can be prepared.
A further object of the invention is to provide hydrothermally stable mesoporous molecular sieve which possesses high thermal stability and surface area for application as catalyst or catalyst support.
Yet another object of the invention is to provide hydrothermally stable mesoporous molecular sieve in which silicon atoms are substituted with other elements to generate active sites.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
Broadly, our invention contemplates the preparation of hydrothermally stable MCM-41 mesoporous molecular sieves.
More specifically, we have found that the hydrothermal stability of MCM-41 mesoporous molecular sieves may be greatly improved by addition of cations like tetra-alkyl ammonium or sodium ions in the synthesis gel. Similar method was also applied for the synthesis of aluminum and other transition metal containing MCM-41 mesoporous molecular sieves. These synthesized materials possess high stability in water at elevated temperatures. Our pure silica and transition metal incorporated MCM-41 molecular sieves are characterized by high surface area in the order of 900to 1100 m2/g. Furthermore, they possess a hexagonal crystal structure as evidenced from the X-ray powder diffraction analysis. Pore size distribution indicates that a large percentage of the pore volume is in the mesopore region, 18 to 100 xc3x85, which is highly desirable for their catalytic application.
Hydrothermally stable MCM-41 mesoporous molecular sieve is prepared by making a gel containing surfactant, inorganic silicate, tetra-alkyl cation and water. The reactants are mixed to provide ratios for the ingredients defined as follows:
surfactant/silica . . . 0.1 to 1
tetra-alkyl cation/surfactant . . . 0 to 2
H20/silica . . . 50 to 250
Si/M . . . xe2x89xa65
Reactant gels defined above are reacted for periods from about 2 hour to 4 days at temperature ranging from 250xc2x0 C. to 60xc2x0 C. In preparing the reaction mixtures, surfactant of the general chemical formula, CnH2n+1(CH3)3N+Xxe2x88x92(where n=12 to 18and x=Cl, Br) was mixed with tetra-alkyl ammonium cation of the general chemical formula, CnH2n+1N+Xxe2x88x92(where n=1 to 3 and X=Cl, Br) before addition of the silicate or metal source.
In a particularly preferred method for preparing hydrothermally stable MCM-41 mesoporous molecular sieves tetra-alkyl ammonium salt solution was slowly mixed with the surfactant solution. The mixture was stirred vigorously for a period of 30 to 60 minutes. To this mixture a solution of inorganic silicate was slowly added and further stirred vigorously for 45-60 minutes. The mixture was then slowly acidified with a dilute mineral acid until pH 9.5 to 10 reached. This gel mixture was aged for 2 hours to 2 days at temperature of 25xc2x0 C. to 600xc2x0 C. Subsequent to aging, the mixture was transferred to polypropylene bottles, sealed and kept at 100xc2x0 C. for 2 to 4 days without agitation. Finally, the bottles were quenched in cold water and the mixture was filtered to recover the solid product which was washed thoroughly with deionized water. To prepare substituted MCM-41 mesoporous molecular sieve suitable metal salt solution was added to the surfactant mixture. Typically, addition of metal salt solution was followed by the addition of silicate solution. The solid material obtained was dried at a temperature of 50 to 70xc2x0 C., followed by calcining at a temperature of 500 to 560xc2x0 C. The final product possesses a high surface area in the range of 1000 to 1200 m2/g and X-ray diffraction analysis shows well defined hexagonal pattern. Pore size distribution indicates that most of the pore volume is in the mesopore region, 18 to 100 xc3x85.
Having described the basic aspects of the present invention the following examples are given to illustrate specific embodiments thereof.